1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives, and more particularly to disk drives that use phase-quadrature servo patterns as part of the head-positioning servo control system.
2. Description of the Related Art
In a magnetic recording hard disk drive, the read/write head is moved across the data tracks of the rotating disk and maintained on the data tracks by an actuator, typically a rotary voice-coil-motor (VCM) actuator. The head detects a position-error-signal (PES) pattern from angularly-spaced servo sectors on the disk and generates a servo signal that is demodulated to provide the PES. The disk drive servo control system receives the PES and generates a control signal to the actuator to maintain the head on track and move it to the desired track for reading and writing of data. Each read/write head is attached to the end of a head carrier or air-bearing slider that rides on a cushion or bearing of air above the rotating disk. The slider is attached to a relatively flexible suspension that permits the slider to “pitch” and “roll” on the air bearing, with the suspension being attached to the end of the VCM actuator arm.
One type of PES pattern is a phase-quadrature pattern that has two generally like patterns radially-spaced in the servo track, but with one pattern phase shifted along-the-track from the other pattern. The pattern is called a quadrature pattern because it includes four bands spaced along-the-track, with the third and fourth bands radially spaced by one-half track from the first and second bands. The conventional servo-signal demodulator for a phase-quadrature pattern provides a PES that is a function of two amplitudes, with the two amplitudes directly corresponding to the servo signals from the two phase-shifted patterns. However, the conventional demodulator does not account for clock errors in reading the servo pattern and for phase-misalignment errors between the two phase-shifted patterns.
What is needed is a disk drive with a phase-quadrature PES pattern and a servo signal demodulator that is insensitive to clock errors and phase misalignment errors.